1. Field of the Invention
The present invention relates to an image reading apparatus.
2. Description of the Related Art
Among conventional image reading apparatuses is an auto-document-feeder type image reading apparatus (hereinafter, “ADF image-reading apparatus”) that can successively read images from sheet-type media (hereinafter, “sheets”) stacked thereon. That is, such an ADF image-reading apparatus can automatically and successively feed a large number of sheets having a predetermined size for reading images therefrom.
Japanese Patent Application Laid-open No. H9-284478 discloses a conventional ADF image-reading apparatus. The conventional ADF image-reading apparatus includes an automatic feeding unit arranged on one side of the conveying unit, and a manual feeding unit arranged on another side of the conveying unit. The conveying unit conveys sheets from the automatic feeding unit and the manual feeding unit to an image reading unit. The conventional ADF-image reading apparatus switches the direction in which a sheet is conveyed (conveying direction) between a normal conveying direction and a reverse conveying direction. The automatic feeding unit automatically separates one sheet at a time from the sheets stacked thereon and feed the separated sheet to the conveying unit. On the other hand, sheets not separatable in the automatic feeding unit (e.g., thin paper sheets or overhead projector (OHP) films) can be manually fed one by one from the manual feeding unit.
To achieve a compact structure, the conventional ADF image-reading apparatus includes a bend section in a conveying path between the automatic feeding unit and the manual feeding unit. However, when, e.g., a comparatively thick card-type medium (hereinafter, “card document”) having higher rigidity is fed from either one of the automatic feeding unit and the manual feeding unit, it is likely that the card document gets damaged by being forced to bend at the bend section. Even if it is assumed that the card document does not get damaged at the bend section, a larger driving force may be necessary to convey it over the bend section. As a result, it becomes difficult to downsize the ADF image reading apparatus. To solve such a problem, a conventional ADF image reading apparatus such as an ADF image reading apparatus 001 described below with reference to FIG. 12 includes a separate card-document feeding unit for feeding exclusively the card documents. Moreover, the conveying path from the card-document feeding unit to a stacking unit is laid in a straight line without any bends therebetween.
The ADF image reading apparatus 001 includes three pairs of rollers 004a, 004b, and 004c, a conveying unit 005, and an ADF unit. The pairs of rollers 004a, 004b, and 004c are arranged symmetrically with respect to the center position of the conveying path for the sheet documents along the width direction of the sheet documents. Each of the pairs of rollers 004a, 004b, and 004c includes a rotatable driving roller 002 and a driven roller 003 that is biased towards and rotates along with the driving roller 002. The ADF unit includes a pick roller 006. The conveying unit 005 conveys a sheet document nipped between the driving roller 002 and the driven roller 003. To make the structure of the apparatus compact, two conveying paths, viz., a conveying path for sheet documents and a conveying path for card documents, are arranged to partially overlap in the width direction. When conveying a sheet document, the pick roller 006 separates a single sheet document at a time and conveys the separated sheet document over the conveying path for sheet documents. A sheet document is conveyed by using all the three pairs of rollers 004a, 004b, and 004c, while a card document is conveyed by using only the pair of rollers 004c arranged on one end.
Because only one pair of rollers 004c is used to convey a card document, it is likely that the card document is not stably conveyed. To solve such a problem, another configuration is implemented in which, e.g., four pairs of rollers 004a, 004b, 004c, and 004d are arranged symmetrically across the center position of the conveying path for the sheet documents along the width direction. In this case, all the four pairs of rollers 004a, 004b, 004c, and 004d are used to convey a sheet document, while two pairs of rollers 004c and 004d arranged side by side on one end are used to convey a card document.
However, in case a sheet document gets skewed before reaching the pairs of rollers 004a, 004b, 004c, and 004d, and if the distance between the farthest pairs of rollers 004a and 004d is large as shown in FIG. 13A, then it is likely that the amount of skew increases because of the large distance between the pairs of rollers 004a and 004d. That is, when a skewed document is conveyed to the pairs of rollers 004a, 004b, 004c, and 004d, the pair of rollers 004a first nips one end of the conveyed sheet document and starts conveying the nipped portion. The pair of rollers 004d nips the other end of the sheet document after the pair of rollers 004b has already conveyed a large portion on one side of the sheet document. Thus, the portion of the sheet document nipped and conveyed by the pair of rollers 004a precedes the portion of the sheet document nipped and conveyed by the pair of rollers 004d by a large margin. As a result, the sheet document spins around in the counterclockwise direction. The amount of increase in skew in the abovementioned configuration is more as compared to a configuration in which two pairs of rollers 004a and 004b are arranged symmetrically across the center position of the conveying path of the sheet documents as shown in FIG. 13B. Moreover, when four pairs of rollers 004a, 004b, 004c, and 004d are arranged, the increase in skew can be prevented only in the case of conveying a card document and not in the case of a sheet document. Thus, it becomes difficult to stably convey both the sheet document and the card document using the same ADF image reading apparatus 001.